A protein phosphatase is an enzyme dephosphorylating a phosphoprotein, and is said to act on the regulation of glucose metabolism, smooth muscle contraction, cell cycle, DNA replication, transcription, translation, cell adhesion, activation and differentiation of cells, etc. in the living body and the maintenance of the immune system and the nervous system of the living body. Accordingly, a protein phosphatase inhibitor has potential to be utilized as various medicines, and thus is widely explored. Among protein phosphatases, a serine/threonine phosphatase (Ser/Thr phosphatase) is classified into four families, PP1, PP2A, PP2B, and PP2C (PPM1), based on its chemical properties and gene structures. The PPM1 family is particularly involved in the regulation of cellular functions such as DNA repair mechanism, stress response, signal transduction, and cell proliferation, and thus the development of an inhibitor of the PPM1 family is particularly gaining attention.
Protein Phosphatase Magnesium-Dependent 1, Delta (PPM1D), belonging to the PPM1 family, is a phosphatase which is induced in a manner dependent on p53, a tumor suppressor protein, and increases in a cell once DNA is damaged by an ultraviolet ray, an electromagnetic ray, and the like. PPM1D is reported to be overly expressed in a plurality of cancer cells including breast cancer (Non Patent Document 1), drawing the attention to the relationship between carcinogenesis and PPM1D. Meanwhile, it is suggested that PPM1D is constitutively expressed in the living body, and based on the studies using knockout mice, PPM1D is involved in not only carcinogenesis but also spermatogenesis, aging, and immune response (Non Patent Document 2).
As described above, despite the fact that some of the protein phosphatases, for example PPM1D, play an important role in carcinogenesis and other phenomena, very little is known about their functions and inhibitors.    Non Patent Document 1: Nat. Genet., 31, 210 to 215, 2002    Non Patent Document 2: Mol. Cell. Biol., 22, 1094 to 1105, 2002